WASHINGTON — Jon Niese took his first significant step in returning from a partially torn left rotator cuff, with a rehab appearance yesterday for the Mets’ rookie-level team in the Gulf Coast league.

The left-hander, who hadn’t seen game action since hitting the disabled list on June 21, allowed four runs, two of which were unearned, on three hits over two innings against the Nationals’ affiliate.

The Mets are hopeful Niese can rejoin the rotation within 2-3 weeks.

“I don’t know how many more [appearances] he’s going to go,” manager Terry Collins said before the Mets’ 4-1 loss to the Nationals. “We’ve got to get him up, [get] some endurance. The one thing we don’t need is to have a pitcher come here this time of year and be a five-inning guy. We’ve got to make sure we build up his pitch count to where he can at least go 100.”

* Kirk Niuewenhuis’ demotion to Triple-A Las Vegas left the Mets without a lefty bat on the bench, but Collins said that is tolerable given the proficiency Justin Turner and Josh Satin have shown against righty pitchers. Turner was available for pinch-hitting duty yesterday, a day after he collided with Andrew Brown on a pop up and later left the game, but didn’t get an at-bat.

* Jeurys Familia, who had surgery in June to remove bone spurs and loose bodies in his right elbow, isn’t necessarily finished for the season, according to Collins. The righty has begun throwing at the Mets’ minor league complex and “made fast strides after surgery.” Familia is a potential September call-up.